In a semiconductor light-emitting device, negative-charge carriers and positive charge carriers, or, technically termed as electrons and holes, are driven (injected) from opposite contacts to meet in the light-emitting region, the so-called active-region. A photon is emitted if an injected electron and an injected hole annihilate with each other in the active-region. This process is called radiative recombination process. The figure-of-merit for the radiative recombination is called internal quantum efficiency (IQE), which equals to the ratio of photons emitted over electron-hole pairs injected. Higher IQE means higher degree of energy savings provided that emitted photons can be properly extracted from the device.
In practice, however, not every injected electron and hole can recombine with each other. Defects (including impurities) in materials of which the active-region is made can often trap electrons or holes before they meet each other. Unwanted electric fields in the active-region can also separate electrons and holes, generating potential barriers preventing radiative recombination. These two facts are often the major roots for IQE reduction. Beside these, improper active-region designs can also lead to reduced IQE. For example, in a very thin active-region (designed so to avoid the large polarization-resulted built-in electric fields), injected carriers can quickly get overcrowded with increasing driving current. Overcrowded electrons (or holes) can scatter each other because of the repulsive force between them. This will reduce the electron-hole pairs' radiative recombination probability. When this happens, it is said Auger effect is taking place. So, Auger effect can be a factor resulting in IQE reduction in very thin active-regions, such as a very thin quantum well.
In nitride semiconductors, material defects and electric fields become more pronounced in terms of IQE reduction. Nitride semiconductors, including InN, GaN, AlN, and any ternary or quaternary of them, can emit light with wavelength from infrared to visible and deep UV range, being considered as the most promising material system for very high-efficiency solid-state lighting. To make a solid-state light-emitting device, such as an LED, different nitride layers need to be incorporated properly to form a structure. This means different nitride layers have to stack on top of other layers. Or, technically, different nitride layers are epitaxially grown on top of other layers. To maintain a good epitaxial growth, it is desired to have the least lattice-constant difference of adjoining layers the best. Lattice mismatch will bring in strain, which can generate defects, resulting in IQE reduction. Unfortunately, nitride semiconductors, especially In-containing nitrides, which are indispensable in realizing solid-state lighting, possess huge lattice mismatch with each other. For example, InN and GaN have a lattice mismatch in c plane above 11%, AlN and GaN mismatch over 3%. To give an example, lattice mismatch of 1% can result in defects (dislocations) of ˜109/cm2. These defects are deleterious when presenting in the device active-region.
C-plane nitrides also possess high-density surface charges, owing to self spontaneous polarization, and strain-induced piezoelectric polarizations. The charge density can be above 1013/cm2, capable of introducing electric field more than 1 MV/cm.
As a result, the state-of-the-art nitride-based LEDs suffer from the abovementioned defects and inner electric fields. One manifestation is the increasing difficulty in making high-In-fraction, and high-Al-fraction LEDs for green and UV LEDs. High-In-fraction and high-Al-fraction means higher strain and higher inner electric fields within the structure. Another one is the LED efficiency droop. It is manifested that the efficiency declines with increasing current, after reaching peak efficiency under a relatively small current. The droop is preventing LEDs from high quantum efficiency under high current density, which is desirable for high power applications such as general lighting.